Electrolytic cell for production of rare earth metals

ABSTRACT

An electrolytic cell for production of rare earth metals is disclosed. The electrolytic cell includes a cell housing provided with one or more inclined channels disposed in a floor of the cell housing along which channel(s) molten rare earth metals produced in the electrolytic cell can drain. One or more cathodes are suspended within the cell housing in substantially vertical alignment with the one or more channels. Respective opposing surfaces of the one or more cathodes are downwardly and outwardly inclined at an angle from the vertical. One or more pairs of anodes are suspended within the cell housing; each anode in the one or more pairs has a facing surface inclined from the vertical and spaced apart in parallel alignment with respective opposing inclined surfaces of the one or more cathodes to define a substantially constant anode-cathode distance therebetween. The electrolytic cell also includes a sump for receiving molten rare earth metals from the channel, wherein the sump is spaced apart and isolated from the one or more cathodes and the one or more anodes. Separation of the molten rare earth metals from the cathode(s) and the anode(s) prevents reaction and/or contamination with fugitive carbon arising from the anode(s) or back reaction with off gases.

FIELD

The present disclosure relates generally to electrolytic cells, inparticular electrolytic cells adapted to produce rare earth metals, suchas neodymium, praseodymium, cerium, lanthanum and mixtures thereof, byan electrolysis process in a molten fluoride or chloride electrolytebath.

BACKGROUND

Electrolytic cells for production of aluminium in a molten fluoride orchloride salt bath are well known and many of their design featuresaddress important considerations. In particular, it is important tomaintain a stable and low anode-cathode distance (ACD) as an energysaving measure in a highly energy intensive process. Maintaining aconstant ACD may prove difficult where molten aluminium pools on thesurface of the cathode and is under hydrodynamic forces imposed bystrong magnetic fields. Accordingly, in some cell configurations, thecathodes may be suspended above the cell floor onto which the moltenaluminium pools. In other configurations, the cathodes may be providedwith channels into which the molten aluminium may collect, therebydraining the molten aluminium from the cathode surface as soon as itforms to maintain a constant ACD.

It is also important that the electrolytic cell is configured toliberate carbon dioxide gas, which evolves at the anode surface duringthe electrolysis, from the interelectrode space to substantially prevent‘back reaction’ with the aluminium metal as it forms on the cathodesurface, thereby reducing the efficiency of the electrolysis process.

Neodymium and praseodymium, mixtures thereof, and other rare earthmetals, are also currently made commercially by an electrolysis processin a molten mixed fluoride salt bath. In contrast to the electrolyticproduction of aluminium, the anodes and cathodes are disposed in avertical orientation and the molten metal is collected into a receivingvessel on the floor of the cell. The interelectrode space is notaffected by the molten metal accumulation, but it is neverthelesssubject to change by the continuous electrolytic consumption of thecarbon anode surfaces. The cathodes are typically comprised of an inertmetal, such as molybdenum or tungsten.

As the anodes are consumed, there is no effective means to keepanode-cathode separation distance uniform. As the major part of theprocess heat is delivered by the ohmic resistance of the electrodespacing, the process temperature is highly variable and generallycontrolled by reduction in current supplied to the cell. This isimpractical in larger scale operations where a number of cells would beconnected in electrical series. Furthermore, deterioration in currentthroughout the electrolysis process is also undesirable since itdecreases the production capacity of the cell. Most importantly, failureto closely control the process temperature reduces the process yield, orFaraday efficiency, and results in the formation of insoluble sludgewhich settles on the floor of the cell. Consequently, the electrolysishas to be periodically halted to remove the sludge from the cell,thereby inhibiting continuous electrolysis.

Poor control of the process temperature also increases the vapouremissions from the cell, which are harmful to the working atmosphere andthe environment if they are not contained.

Additionally, as the anodes are consumed, their displaced volume in theelectrolyte decreases and the electrolyte level in the cell falls. Thisreduces the working area of the anode immersed in the electrolyte, tothe detriment of process efficiency including power consumption andincreased possibility of ‘anode effects’ generating highly pollutinggases.

Moreover, the product rare earth metal is reactive with carbon at theprocess temperature. Carbon is a highly undesirable impurity for certainrare earth metal product applications. Decreasing the possibility ofcontact between fugitive carbon in the cell and the metal and/or theresidence time of product metal in the cell are desirable designattributes that are not apparent in the current commercial cell designs.This particular problem is not a factor in the design of electrolyticcells for aluminium production because aluminium does not react withcarbon under these conditions.

Additionally, in current electrolytic cell designs for rare earthmetals, it is difficult to maintain the product rare earth metals in amolten state because the operating temperatures are preferably only10-30° C. above the freezing point of the product rare earth metals.This problem is not an issue and is not addressed in electrolytic cellsfor electrolytic production of aluminium because the process temperatureis about 300° C. above the freezing point of aluminium.

Current commercial activities for electrolytic production of rare earthmetals are small in scale, labour intensive and operated in a semi-batchmanner. Several deficiencies prevent the process from being scaled up toallow higher productivity, continuous electrolysis, and high standardsof environmental performance, occupational health and safety to beachieved.

Firstly, the electrolysis cells generally operate in a limited currentrange of 5-10 kiloamperes, commensurate with low production capacity.

There is poor control of a rare earth oxide feed material to the cell,resulting in the accumulation of insoluble sludges that require frequentcell clean-out thereby hindering continuous electrolysis. Additionally,feed material is delivered to the cell manually, without a knownreference to the current oxide concentration in the cell.

The existing technology uses vertical electrode arrangements. Sucharrangements are not amenable to achieving a high Faraday efficiency.For example, gas bubbles which evolve and rise from the anode surfaceare likely to be entrained in the electrolyte flows and make contactwith the product metal forming on the cathode plates, thereby reducingthe process yield consequent to back-oxidation of the product metal.

Keller in U.S. Pat. No. 5,810,993 describes a method of producingneodymium in an electrolytic cell designed to operate without theoccurrence of anode effects, therefore avoiding the generation andrelease of highly polluting perfluorinated carbon (PFC) gases. In thisinvention, the objectives are achieved firstly by providing a multitudeof anode plates such that the anodic current density remains well belowthat at which the anode effect may occur, and secondly by physicallyseparating the vertical cathodes from the vertical anodes using an inertbarrier material which remains porous to neodymium ions, such that ahigher concentration of dissolved neodymium oxide can be maintained inthe anode region than in the cathode region. The disclosed invention hasa number of deficiencies and impracticalities however. There is nodemonstration in the cited examples that the barrier material (boronnitride) is indeed permeable to neodymium ions as would be required fora continuous electrolysis process. Further, the proposed anode design iscomplex and the wear rate of the anode plates may be expected to behighly non-uniform and wasteful. The compartmental separation of theanodic and cathodic zones further results in a large interelectrodeseparation distance, and a resulting inefficient energy consumption.Further, the invention proposes use of carbon as the inert cathodematerial, while it is well known that carbon will react with andcontaminate the product metal.

There is therefore a need for alternative or improved electrolytic cellsand processes for producing rare earth metals.

The above references to the background art do not constitute anadmission that the art forms a part of the common general knowledge of aperson of ordinary skill in the art. The above references are also notintended to limit the electrolytic cell as disclosed herein.

SUMMARY OF THE DISCLOSURE

In a first aspect there is disclosed an electrolytic cell for productionof rare earth metals comprising:

-   -   a cell housing provided with one or more inclined channels        disposed in a floor of the cell housing along which channel(s)        the molten rare earth metals produced in the electrolytic cell        can drain;    -   one or more cathodes suspended within the cell housing in        substantially vertical alignment with the one or more channels,        respective opposing surfaces of the one or more cathodes being        downwardly and outwardly inclined at an angle from the vertical;    -   one or more pairs of anodes suspended within the cell housing,        each anode in the one or more pairs having a facing surface        inclined from the vertical and spaced apart in parallel        alignment with respective opposing inclined surfaces of the one        or more cathodes to define a substantially constant        anode-cathode distance therebetween; and,    -   a sump for receiving molten rare earth metals from the channel,        wherein the sump is spaced apart and isolated from the one or        more cathodes and the one or more pairs of anodes suspended        within the cell housing.

In a second aspect there is disclosed an electrolytic cell forproduction of rare earth metals comprising:

-   -   a cell housing for containing an electrolyte bath;    -   one or more cathodes suspended within the cell housing;    -   one or more pairs of consumable anodes suspended within the cell        housing, each anode in the one or more pairs being spaced apart        from respective opposing sides of the cathode; and    -   a displacement device to control a height of the electrolyte        bath contained in the cell housing.

In one embodiment said displacement device controls the height of theelectrolyte bath contained in the cell housing in response to anodeconsumption and a volume of rare earth metal product contained in thecell housing.

In a third aspect there is disclosed an electrolytic cell for productionof rare earth metals comprising:

-   -   a cell housing;    -   one or more cathodes suspended within the cell housing;    -   one or more pairs of consumable anodes suspended within the cell        housing, each anode in the one or more pairs being spaced apart        from respective opposing sides of the cathode; and,    -   a device operatively associated with the one or more pairs of        consumable anodes to control a distance between the anodes and        the cathode in response to anode consumption.

In a further aspect there is disclosed a system for electrolyticallyproducing rare earth metals comprising:

-   -   an electrolytic cell in accordance with any one of the first,        second or third aspects as defined above;    -   a feed material comprising one or more rare earth metal        compounds capable of undergoing electrolysis to produce rare        earth metals;    -   an electrolyte in which molten state the feed material is        soluble; and,    -   a source of direct current configured to pass a current between        an anode and a cathode of the electrolytic cell to electrolyse        the feed material and thereby produce molten rare earth metal        product.

In another aspect there is disclosed a process for electrolyticallyproducing rare earth metals comprising:

-   -   providing an electrolytic cell in accordance with the second        aspect;    -   charging the electrolytic cell with a feed material comprising        one or more rare earth metal compounds capable of undergoing        electrolysis to produce rare earth metals and an electrolyte        bath comprising molten electrolyte in which the feed material is        soluble;    -   passing a direct current between at least one consumable anode        and a cathode in the electrolytic cell to electrolyse the feed        material and thereby produce molten rare earth metal product;        and,    -   displacing the molten electrolyte in the electrolytic cell to        maintain a height of the electrolyte bath in the electrolytic        cell.

In one embodiment, the step of displacing is performed in response to arate of anode consumption and/or a change in a volume of rare earthmetal product contained in the electrolytic cell.

In a still further aspect there is disclosed a process forelectrolytically producing rare earth metals comprising:

-   -   providing an electrolytic cell in accordance with the third        aspect;    -   charging the electrolytic cell with a feed material comprising        one or more rare earth metal compounds capable of undergoing        electrolysis to produce rare earth metals and a molten        electrolyte in which the feed material is soluble;    -   passing a direct current between at least one consumable anode        and a cathode in the electrolytic cell to electrolyse the feed        material and thereby produce molten rare earth metal product;        and,    -   translating the or each consumable anode toward the cathode in        response to a rate of anode consumption to maintain a constant        anode-cathode distance in the electrolytic cell.

Embodiments disclosed allow improved control capability foranode-cathode distance (ACD) and consequently process temperature,improved control of electrolyte bath height in the electrolytic cell andanode immersion, better mixing of the electrolyte to enhance dissolutionof the feed material, and higher Faraday efficiency by limitingopportunity for back reaction of anode gas with produced metal.

BRIEF DESCRIPTION OF THE FIGURES

Notwithstanding any other forms which may fall within the scope of thedisclosure as set forth in the Summary, specific embodiments will now bedescribed, by way of example only, with reference to the accompanyingdrawings in which:

FIG. 1 is side view of an electrolytic cell in accordance with onespecific embodiment; and

FIG. 2 is a cross-sectional view of the electrolytic cell shown in FIG.1.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The description broadly relates to an electrolytic cell arranged toproduce rare earth metals by an electrolysis process in a moltenelectrolytic salt bath.

The rare earth metals produced in the electrolytic cell disclosed hereininclude those rare earth metals having a melting point less than 1100°C. Exemplary rare earth metals include, but are not limited to, Ce, La,Nd, Pr, Sm, Eu, and alloys thereof including didymium and mischmetal.The electrolytic cell disclosed herein is also suitable for theproduction of alloys of rare earth metals with iron.

The molten electrolytic salt bath behaves as a solvent for the feedmaterial. The electrolyte for use in the molten electrolytic salt bathmay comprise halide salts, in particular fluoride salts. Examples of‘fluoride salts’ include, but are not limited to, metal fluoride saltsincluding rare earth metal fluorides such as LaF₃, CeF₃, NdF₃, and PrF₃,alkali metal fluorides such as LiF, KF, and alkaline earth metalfluorides such as CaF₂, BaF₂.

Selection of a feed material for the electrolysis process will depend onthe desired rare earth metal product and the composition of theelectrolyte. Where the electrolyte is composed of fluoride salts, thefeed material that is subjected to the electrolysis process may compriseoxides of the rare earth metals.

The term ‘rare earth metal oxide’ broadly refers to any oxide or anyprecursors of such oxides of a rare earth metal, including rare earthmetal hydroxides, carbonates or oxalates. Rare earth metals are a set ofseventeen chemical elements in the periodic table, specifically thefifteen lanthanides plus scandium and yttrium. Scandium and yttrium areconsidered rare earth metals since they tend to occur in the same oredeposits as the lanthanides and exhibit similar chemical properties. Thelanthanides include lanthanum, cerium, praseodymium, neodymium,promethium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium, and lutetium.

Suitable examples of feed material for electrolytic production ofneodymium or praseodymium include neodymium oxide (Nd₂O₃) orpraseodymium oxide (Pr₆O₁₁). Where an alloy, such as didymium, is thedesired product the feed material may comprise two or more oxides ofrare earth metals (e.g. Nd₂O₃ and Pr₆O₁₁) in the desired stoichiometricratio of the desired alloy. Mischmetal may be prepared from oxides ofseveral rare earth metals, such as Ce, La, Nd, Pr, wherein the ratio ofrare earth metals in the mischmetal corresponds to the ratio of rareearth metal oxides in the feed material.

Alternatively, where the electrolyte is composed of chloride salts, thefeed material may comprise chloride salts of the rare earth metals.

In one embodiment the electrolyte comprises one or more rare earth metalfluorides and lithium fluoride. The one or more rare earth metalfluorides may be present in the electrolyte in a range of about 70-95 wt% with the balance as lithium fluoride. Optionally, the electrolyte mayfurther comprise up to 20 wt % calcium fluoride and/or barium fluoride.

It will be appreciated by persons skilled in the art that the operatingtemperature of the electrolytic cell will depend on the target rareearth metal product or rare earth metal alloy, the composition of theelectrolyte, and consequently the respective freezing points of the rareearth metal, alloy and electrolyte. In one embodiment, the operatingtemperature of the electrolytic cell may be in the range of 5-50° C.above the freezing point of the electrolyte, and preferably 10-20° C.above the melting point of the electrolyte. The composition of theelectrolyte is selected so that the liquidus of the electrolyte may bein a range of 5-50° C. above the freezing point of the metal.

In some embodiments, where the target rare earth metal product ismischmetal (a mixture of cerium, lanthanum, neodymium and praseodymium),the freezing point is variable depending on the composition of themischmetal and the relative ratios of the rare earth metals therein, butnonetheless is around 800° C. In these embodiments, the electrolyte mayinclude barium or calcium fluorides as described above to achieve anelectrolyte liquidus in the range of 5-50° C. above the freezing pointof the mischmetal.

In other embodiments, where the freezing points of the rare earth metalalloys or mixtures are 800° C. or lower, the electrolyte may optionallycomprise one or more rare earth metal chloride and lithium chloridesalts.

Referring to FIGS. 1 and 2, where like numerals refer to like partsthroughout, there is shown an embodiment of an electrolytic cell 10 forproduction of rare earth metals. The cell 10 includes a housing 12having a floor 14, a sump 16, one or more cathodes 18, and one or morepairs of anodes 20.

The housing 12 is formed from anti-corrosive materials which are inertin view of the electrolyte composition and operating conditions, as hasbeen described in the preceding paragraphs. In particular, theanti-corrosive materials used to internally line the housing 12 shouldbe resistant to forming an alloy with the rare earth metals producedtherein. In one embodiment the housing 12 may be lined internally withrefractory materials. Suitable refractory materials include, but are notlimited to, carbon, silicon carbide, silicon nitride, boron nitride, orcertain stainless steels such as will be well known to those skilled inthe art.

The inclined floor 14 has one or more inclined channels 22 disposedtherein along which molten rare earth metals produced in theelectrolytic cell 10 can drain. In one embodiment, the one or moreinclined channels 22 are inclined from the horizontal at an angle α ofup to about 10°.

In the embodiment shown in FIG. 2, the channel 22 has a rectangularcross-section. It will be appreciated, however, that in alternativeembodiments, the cross-section of the channel 26 may take other forms,such as a V-shape or a U-shape.

In some forms of the invention the floor 14 may be provided with morethan one inclined channel 22, as shown in FIG. 2. In these particularforms the channels 22 are configured in adjacent lateral parallelalignment with one another. In general, the channel(s) 22 may be alignedalong or spaced equidistantly from a central longitudinal axis of thefloor 14 in the housing 12. In this arrangement, the channel(s) 22 inthe floor 14 may be located proximal to an underside 24 of the one ormore cathodes 18 to receive molten rare earth metals produced on the oneor more cathodes 18.

The floor 14, or an upper surface of the floor 14, may be formed fromanti-corrosive materials similar to or the same as those materialsselected for the lining of the cell housing 12. All surfaces havingdirect contact with the rare earth metal product, including thechannel(s) 22 and the sump 16 should be resistant to forming alloys withthe rare earth metals produced in the electrolytic bath. Suitable liningmaterials for the channel(s) 22 and the sump 16 include, but are notlimited to, metals such as tungsten, molybdenum, or tantalum.

The sump 16 is configured to receive, in use, molten rare earth metalproduced on the one or more cathodes 18 which collects in the channeland drains towards the lower end 26 of the channel 22. The sump 16 isspaced apart and isolated from the one or more cathodes 18 and the oneor more anodes 20.

The sump 16 may be provided with a heater to maintain a temperatureabove the liquidus of the molten rare earth metal. The sump 16 may alsobe provided with a port (not shown) from which molten rare earth metalmay be tapped as required. The sump 16 may be formed from inert metalssimilar to those used for the housing 12.

The arrangement allows for continuous removal of molten rare earth metalproduct from the floor 14 of the cell 10 which prevents pooling of themolten rare earth metal product and consequently provides severaladvantages. In prior art electrolytic cells where a pool of molten rareearth metal product is allowed to form, particularly on the floor of thecell or at a cathodic surface, it is common for the molten rare earthmetal product to become contaminated with ‘sludge’ which comprisesundissolved and partially molten rare earth feed material, reactionintermediates, and byproducts. In the electrolytic cell 10 disclosedherein, in the absence of molten rare earth metal product, the sludgeremains in contact with the molten electrolyte and is thereby providedwith an opportunity for re-dissolution in the molten electrolyte.

The molten rare earth metal product collected in the sump 16 is spacedapart from and isolated from the one or more cathodes 18 and the one ormore anodes 20. Consequently, the molten rare earth metal is protectedfrom reaction and/or contamination with fugitive carbon arising from theone or more anodes 20, and back reactions with off gases from the one ormore anodes 20.

The one or more cathodes 18 are suspended in the electrolyte bath 11contained within the cell housing 12 above the channel 22 insubstantially vertical alignment therewith. In the form as illustrated,the cathodes 18 comprise plates of cathodic material having an uppersurface 28 and opposing elongate surfaces 30, with the underside 24being disposed above the channel 22 in so that molten rare earth metalproduced on the opposing surfaces 30 may fall under gravity directlyinto the underlying channel 22. The opposing surfaces 30 of the cathodes18 are supported by an inert refractory filler material 32 which furtheravoids the formation of an inactive electrolyte zone in the cell 10.

The cathodes 18 are configured in adjacent alignment with one anotherwhereby opposing elongate surfaces 30 of adjacent cathodes 18 arerespectively longitudinally aligned with one another and respectiveopposing end surfaces of adjacent cathodes 18 face one another. It willbe appreciated by persons skilled in the art that spacing between facingopposing end surfaces of adjacent cathodes 18 is as narrow as possible.

The plates of cathodic material are correspondingly sized so that, inthe arrangement as described above, an effective length of theadjacently disposed cathodes 18 is substantially the same as ormarginally shorter than the length of the channel 22.

Alternatively, a single cathode 18 having a similar length as thechannel 22 may be employed in the electrolytic cell 10 as disclosedherein.

The opposing elongate surfaces 30 of the cathodes 18 are downwardly andoutwardly inclined at an angle from the vertical, whereby across-sectional shape of the cathode 18 is substantially triangular. Theopposing elongate surfaces 30 may be inclined from the vertical by angleβ of up to about 45°, and preferably from 2° to 10°.

The angle of inclination is selected on the basis of optimisedbubble-driven flow of electrolyte to achieve good mixing with feedmaterial, and maintenance of high Faraday yield. The desired angle β maybe determined by computational modelling for the specific cell geometry.

In embodiments where a single rare earth metal or an alloy of rare earthmetals is the desired electrolytic product, the cathodes 18 may beformed from an electrically conductive material with sufficientresistive heat properties to ensure free flow of the molten rare earthmetals at temperatures marginally greater than their melting points.Such materials should be resistant to forming alloys with the rare earthmetals produced in the electrolytic bath. Suitable materials include,but are not limited to, metals such as tungsten, molybdenum, ortantalum.

In alternative embodiments where an alloy of iron with one or more rareearth metals is desired, the cathode 18 may be formed from iron. It willbe appreciated by persons skilled in the art that in these particularembodiments, the cathode 18 will be consumed during the electrolyticprocess for production of the iron-rare earth metal alloy.

In the embodiment shown in FIGS. 1 and 2, a plurality of pairs of anodes20 are suspended within the cell housing 12. Each anode 20 in the pairis spaced apart from respective opposing elongate surfaces 30 of thecathodes 18. In the form as illustrated, the anodes 20 comprise platesof consumable anodic material having an upper surface 32, a lowersurface 34, opposing distal and proximal elongate surfaces 36 a, 36 band opposing ends 38. Distal elongate surface 36 a of each anode 20 maybe substantially vertical or may be inclined from the vertical. Theproximal elongate surface 36 b is inclined from the vertical. Theproximal elongate side 36 b may be inclined from the vertical by angleβ′ of up to about 45°, and preferably from 2° to 10°, tapering towardthe lower surface 34 of the anode 20.

The proximal elongate surfaces 36 b of the anodes 18 face respectiveopposing elongate surfaces 30 of the cathodes 18. Both surfaces 36 b and30 are inclined from the vertical by corresponding angle β′ such thatthe said surfaces 36 b and 30 are spaced apart in parallel alignmentwith one another so as to define a substantially constant anode-cathodedistance therebetween.

The anodes 20 are configured in adjacent alignment with one anotherwhereby opposing elongate surfaces 36 a, 36 b of adjacent anodes 20 arerespectively longitudinally aligned with one another and respectiveopposing ends 38 of adjacent anodes 20 face one another. It will beappreciated by persons skilled in the art that spacing between facingopposing ends 38 of adjacent anodes 20 is as narrow as possible.

The plates of anodic material are correspondingly sized so that, in thearrangement as described above, an effective length of the adjacentlydisposed anodes 20 is substantially the same as or marginally shorterthan the length of the channel 22.

Alternatively, a single pair of anodes 20 having a similar length as thechannel 22 may be employed in the electrolytic cell 10 as disclosedherein.

Suitable examples of consumable anodic material include, but are notlimited to, carbon-based materials in particular high purity carbon,electrode grade graphite, calcined petroleum coke-coal tar pitchformulations. Such formulations will be well known to those skilled inelectrolytic production of rare earth metals and other metals such asaluminium.

The anodes are consumed as the electrolysis process progresses and theangle of inclination β of proximal elongate side 36 b remainssubstantially constant. Gas bubbles released from the anode 20 aretherefore retained close to the proximal elongate surface 36 b as thegas bubbles rise to the electrolyte surface, by virtue of the inclinedprofile of proximal elongate surface 36 b, as illustrated in FIG. 2.Advantageously, this reduces the opportunity for contact of the evolvedgas with metal forming on the cathode 18, hence improving Faradayefficiency and avoiding insoluble sludges formed by back reactiontherewith.

Under most operating conditions the ACD in the electrolytic cell, asdisclosed herein, may be between about 30 mm to about 200 mm, althoughan ACD of between about 50 mm to about 100 mm is preferred. The personskilled in the art may readily determine an appropriate ACD depending onthe desired heat generation in the electrolyte zone, electrolyte flowsfor optimum solubility of the feed material, and optimisation of theprocess yield (Faraday efficiency).

The anode is consumed during electrolysis and consequently the ACD mayincrease as electrolysis progresses. The electrolysis cell 10 disclosedherein may be provided with a device 40 operatively associated with theone or more anodes 20 to control the ACD, in particular to maintain asubstantially constant ACD. Said device 40 may comprise a horizontalpositioning apparatus in operative communication with the one or moreanodes 20. In use, the horizontal positioning apparatus may laterallytranslate the one or more anodes 20 toward the cathode 18 in response toa rate at which the anode 20 is consumed so that the ACD may remainsubstantially constant. The rate of anode consumption may be determinedby reference to current flow. Alternatively, the horizontal positioningapparatus may translate the one or more anodes 20 in response tovariation in cell resistance from a predetermined value.

Consequent to anode consumption, the volume occupied by the anodes 20 inthe electrolytic cell 10 decreases thereby lowering the height of theelectrolyte bath in the housing 12. Similarly, the intermittent celloperations such as the replacement of spent anodes with new anodes, andthe removal of rare earth metal product from the cell, will result insubstantial and undesirable variation in the height of the electrolytebath and the electrode immersion depth. The electrolysis cell 10disclosed herein may be provided with a displacement device 42 tocontrol the height of the electrolyte bath in the housing 12, inparticular to maintain a substantially constant height of theelectrolyte bath in the housing 12. The displacement device 42 maycomprise an inert body which is suspended in the housing 12 andpositionable in a vertical direction. In use, the inert body may bedownwardly or upwardly translated in response to specific cell operationso that the height of the electrolyte bath may remain substantiallyconstant. The inert body may take any suitable form, for example a baras illustrated in the Figures.

The displacement device 42 may formed from similar refractory materialsas the inner linings of the housing 12 as described previously.

In use, the electrolysis process may be performed by charging the moltenelectrolyte to the electrolytic cell 10 as described herein. Analternating current may be supplied between the cathodes 18 and theanodes 20 and the resistance of the electrodes 18, 20 raises theoperating temperature of the electrolytic cell 10 to a predeterminedtemperature. The feed material is then charged to the electrolytic cell10 and dissolves in the molten electrolyte. A direct current in a rangeof 5-100 kiloamperes is supplied to the anodes 20, whereuponelectrolysis of the dissolved feed material commences. In theelectrolytic reaction the feed material is reduced to molten rare earthmetal(s) on the opposing elongate surfaces 30 of the cathode 18. Themolten rare earth metal(s) subsequently fall into the channel 22 anddrain along the channel 22 into the sump 16, which is tapped asrequired. Feed material may be regularly charged to the electrolyticcell 10 into areas of high electrolyte flow, at a rate correspondingmore or less to the consumption rate. It will be appreciated by thosefamiliar with the art that the feed rate may be finely controlled toachieve a target cell resistance corresponding to the desiredconcentration of feed in the electrolyte.

The electrolysis process may be performed under an inert or low oxygenatmosphere within the electrolytic cell 10. The inert atmosphere may beestablished and maintained by supplying an inert gas or gas mixtures tothe electrolytic cell 10 to exclude air therefrom and thereby preventundesirable reactions with the molten electrolyte and/or the electrodes18, 20. Suitable examples of inert gases include, but are not limitedto, helium, argon, and nitrogen.

Numerous variations and modifications will suggest themselves to personsskilled in the relevant art, in addition to those already described,without departing from the basic inventive concepts. All such variationsand modifications are to be considered within the scope of the presentinvention, the nature of which is to be determined from the precedingdescription.

In the claims which follow, and in the preceding description, exceptwhere the context requires otherwise due to express language ornecessary implication, the word “comprise” and variations such as“comprises” or “comprising” are used in an inclusive sense, i.e. tospecify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments of theapparatus and method as disclosed herein.

1-17. (canceled)
 18. An electrolytic cell for production of rare earthmetals comprising: a cell housing provided with one or more inclinedchannels disposed in a floor of the cell housing along which channel(s)molten rare earth metals produced in the electrolytic cell can drain;one or more cathodes suspended within the cell housing in substantiallyvertical alignment with the one or more channels, respective opposingsurfaces of the one or more cathodes being downwardly and outwardlyinclined at an angle from the vertical; one or more pairs of anodessuspended within the cell housing, each anode in the one or more pairshaving a facing surface inclined from the vertical and spaced apart inparallel alignment with respective opposing inclined surfaces of the oneor more cathodes to define a substantially constant anode-cathodedistance therebetween; and, a sump for receiving molten rare earthmetals from the channel, wherein the sump is spaced apart and isolatedfrom the one or more cathodes and the one or more anodes.
 19. Theelectrolytic cell as defined in claim 18 further comprising adisplacement device to control a height of the electrolyte bathcontained in the cell housing.
 20. The electrolytic cell as defined inclaim 18 further comprising a device operatively associated with the oneor more anodes to control a distance between the anodes and the opposingsides of the cathode in response to anode consumption.
 21. Theelectrolytic cell as defined in claim 19, wherein displacement devicecomprises an inert body which is suspended in the housing andpositionable in a vertical direction.
 22. The electrolytic cell asdefined in claim 20, wherein the device operatively associated with theone or more anodes comprises a horizontal positioning apparatus.
 23. Theelectrolytic cell as defined in claim 22, wherein the horizontalpositioning apparatus is configured, in use, to laterally translate theone or more anodes towards the cathode in response to a rate at whichthe anodes are consumed.
 24. The electrolytic cell as defined in claim18, wherein the one or more channels therein are inclined from thehorizontal at an angle of up to about 10°.
 25. The electrolytic cell asdefined in claim 18, wherein the one or more channels have across-sectional shape that is rectangular, V-shaped or U-shaped.
 26. Theelectrolytic cell as defined in claim 18, wherein the opposing sides ofthe cathode and the facing sides of the anode are inclined from thevertical by up to 45°.
 27. The electrolytic cell as defined in claim 26,wherein the opposing sides of the cathode and the facing sides of theanode are inclined from the vertical by 2° to 10°.
 28. A system forelectrolytically producing rare earth metals comprising: an electrolyticcell as defined in claim 18; a feed material comprising one or more rareearth metal compounds capable of undergoing electrolysis to produce rareearth metals; a molten electrolyte in which the feed material issoluble; and, a source of direct current configured to pass a currentbetween an anode and a cathode in the electrolytic cell to electrolysethe feed material and thereby produce molten rare earth metal product inthe electrolytic cell.
 29. A process for electrolytically producing rareearth metals comprising: providing an electrolytic cell as defined inclaim 19; charging the electrolytic cell with a feed material comprisingone or more rare earth metal compounds capable of undergoingelectrolysis to produce rare earth metals and an electrolyte bathcomprising molten electrolyte in which the feed material is soluble;passing a direct current between at least one consumable anode and acathode in the electrolytic cell to electrolyse the feed material andthereby produce molten rare earth metal product on the cathode; and,displacing the molten electrolyte in the electrolytic cell to maintain aheight of the electrolyte bath in the electrolytic cell.
 30. A processfor electrolytically producing rare earth metals comprising: providingan electrolytic cell according to claim 20; charging the electrolyticcell with a feed material comprising one or more rare earth metalcompounds capable of undergoing electrolysis to produce rare earthmetals and a molten electrolyte in which the feed material is soluble;passing a direct current between at least one consumable anode and acathode in the electrolytic cell to electrolyse the feed material andthereby produce molten rare earth metal product on the cathode; and,translating the or each consumable anode toward the cathode in responseto a rate of anode consumption to maintain a constant cathode-anodedistance in the electrolytic cell.